Vaccination of Wild Mice Could Reduce Transmission of Lyme Disease

Vaccinating large populations of wild mice against the bacterium that causes Lyme disease could one day help reduce the risk of transmission of the infection to horses.

A recent study, supported by the National Institute of Allergy and Infectious Diseases, demonstrated that the vaccination of wildlife hosts might be a promising, ecologically based strategy to help prevent the spread of vector-borne diseases like Lyme disease and West Nile virus.

Lyme disease is caused by Borrelia burgdorferi, a spiral-shaped bacterium spread through the bite of a blacklegged (Ixodes) tick. It is estimated that approximately 50% of adult horses in areas of the northeastern United States have been infected, but fewer than 10% develop clinical signs of the disease. There is no commercially available equine Lyme disease vaccine.

For the study, researchers trapped more than 900 white-footed mice in 12 different forested sites in Connecticut. The mice were immunized with a Lyme vaccine that is currently used in dogs.

"One of our goals," said study collaborator Durland Fish, PhD, professor of epidemiology at Yale University, "was to keep the infection out of the tick population."

When a Lyme-infected tick feeds on a vaccinated mouse, the rodent's antibodies kill the bacterium inside the tick, preventing the tick from spreading the disease to its next host. The white-footed mouse is known to be a key animal reservoir for Lyme disease.

The researchers thought that by immunizing a large sample of the mice, fewer ticks would become infected, and fewer would be capable of transmitting the disease to humans. "This theory could be extended to include the equine population, as well," Fish noted.

After vaccinating roughly 55% of the mouse population in targeted areas, researchers measured an overall reduction in the prevalence of Lyme disease infection in nymph-stage ticks. Although the results of the study are promising, the scientists don't expect to see mouse-immunization programs implemented any time soon.

According to the study's lead investigator, Alan Barbour, MD, of the University of California, Irvine, "We first need to determine if this vaccine will work when given orally, by pellets or bait. It just isn't feasible to vaccinate large populations of mice by injection."

Once this challenge is addressed, he noted, large-scale immunization programs would require substantial funding, and could take years to develop.

In the meantime, horse owners should take a pro-active approach, advised Sandra Bushmich, DVM, professor of pathobiology and veterinary science at the University of Connecticut. "Horse owners can help prevent Lyme disease by performing thorough tick checks on a regular basis," she said. "It's also important to apply a permethrin-based tick repellent on horses. These products should be used in the spring and fall, as well as in the summer months."

About the Author

Rallie McAllister, MD, grew up on a horse farm in Tennessee, and has raised and trained horses all of her life. She now lives in Lexington, Ky., on a horse farm with her husband and three sons. In addition to her practice of emergency and corporate medicine, she is a syndicated columnist (Your Health by Dr. Rallie McAllister), and the author of four health-realted books, including Riding For Life, published by Eclipse Press and available at www.ExclusivelyEquine.com or by calling 800/582-5604.

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